The present invention relates to electromechanical actuators having non-dynamic or pseudo-static driving mechanisms and the control and driving of such actuators.
The small motor market has been increasing continuously for decades and there is a particular interest in high performance miniature motors that can be fabricated at low costs. Force and precision are the typical main properties of importance. Common electric motors have in some applications reached their limits and alternatives are being developed. This invention relates to the need for high performance miniature motors
Electromechanical motors, comprising e.g. piezoelectric motors, is a more and more used type of miniature motors. Piezoelectric actuators are today well known and used in a wide variety of applications. Piezoelectric actuators are generally characterised by a high momentum but a small stroke. By repeating the motion with a high frequency, macroscopic strokes can be achieved. There are a number of fundamentally different operation mechanisms used in electromechanical motors. By using the inertia of some motor component and/or time dependent physical effects, various electromechanical motors can be realized. This group of driving mechanisms may be referred to as dynamic driving mechanisms. Typically, motors with dynamic driving mechanisms can only operate in a certain frequency range, while no operation at low internal speed or frequency is possible. The commonly encountered names ultrasonic and travelling wave motors belong to this group of dynamic driving mechanisms.
Another group of driving mechanisms can be denoted as non-dynamic, static or quasi-static. These non-dynamic mechanisms are characterised in that the motion can be made at arbitrarily low frequencies or speed of the active elements. The driven component is moved by actuator elements which typically make motion cycles such as grip, move, release and return. When one set of elements is releasing, another set of elements will grip the driven component. Typically, the non-dynamic mechanisms are advantageous where controlled positioning is desired at low to medium-high speeds. Further this mechanism allows for easy optimisation in various applications and gives the possibility to deliver high forces. The main disadvantage is the high demands on the construction in order to achieve the desired mechanism. Various solutions to simplify the constructions without losses in performance are therefore generally of great commercial interest.
One mechanisms for non-dynamic motion is the xe2x80x9cinchwormxe2x80x9d mechanism, first disclosed in the U.S. Pat. No. 3,902,084. The driven component is moved by mechanical steps in a clamp-extend-unclamp fashion, e.g. in U.S. Pat. No. 5,751,090. There has to be at least two sets of clamping elements that move out of phase. In between each motion, the extention, the driven components is clamped by both sets of elements and stands still. The motion is cyclic and the ultimate resolution corresponds to one step length divided by the voltage resolution. The driven component can in some cases be stopped at fractions of the full step length, a kind of micro-step mode. The clamping and unclamping takes place during a non-motion phase.
In the international patent application WO 97/36366 a piezoelectric motor based on a non-dynamic driving mechanism is disclosed. The mechanism is an alternative to the xe2x80x9cinch-wormxe2x80x9d mechanism and could be denominated a xe2x80x9cmechanical stepping mechanismxe2x80x9d. The motor is made of an electromechanical material as a monolithic multilayer unit with at least two independent sets of drive elements that can move two-dimensionally. The motion of each set is characterised by the four sequences of gripping, moving, releasing and returning. Voltages cycles are applied to the sets of bimorph drive elements, which are out of phase with each other. In the application the preferred voltage cycles were stated to be sinusoidal.
Prior art non-dynamically driven electromechanical motors exhibit large advantages. However, some minor disadvantages are still present. The drive elements typically have a strong coupling between supplied/removed charge and the mechanical shape changes. One way in prior art to control the voltages over the drive elements is to connect a voltage amplifier connected substantially directly to the drive elements. In this amplifier circuit, the drive elements are driven by voltage control, which from the drive elements capacitance point of view is electrically analogous to a closed circuit. If a force is applied on the drive element, the electromechanical material will give rise to a voltage or current. A closed circuit will compensate for this by adjusting the voltage to the requested value. In practise, this means that the efficient Youngs modulus of the electromechanical material will be quite low due to the strong coupling between the mechanical shape and the low impedance of the amplifier allowing the electrical charge to be removed without noticable resistance. A higher Youngs modulus is normally advantageous since higher forces, higher frequencies and smaller sizes can be accomplished,
Furthermore, the size of an amplifier circuit is often quite large, when discussing modem electronics, and since electromechanical motors often are used in miniature applications, this size can correspond to a substantial part of the total size. The power dissipation of an amplifier circuit is typically relatively large, which influences the need for cooling arrangements. This results in larger size motors. Amplifier cirucits are also relatively costly.
In ultrasonic motors the wear of the contact surfaces is a non-negligible problem. Several solutions to the problems have been suggested including polymer surface and lubrication. The wear of a non-resonant motor is less due to the more controlled motion of the drive elements. However, when high performance miniature motors are considered, also a minor wear might affect the performance.
There are numerous ways to make piezoceramic motors according to the present invention but with prior art solutions it is difficult to achieve small size, high forces and low price at the same time.
A general object of the present invention is to provide an improved control device and control method for electromechanical motors, having a non-dynamic or pseudo-static driving mechanism. A further object is to increase the apparent Youngs modulus for the material in the drive elements. Another object is to provide a motor, which is smaller, withstands higher forces and has less power consumption.
The above objects are achieved by devices and methods according to the enclosed claims. In general words, an electromechanical actuator arrangement and a driving device for such an arrangement, having a plurality of drive element to be driven according to a walking mechanism is provided. The driving device is characterised by an electrical power source and at least two switches connected in series between the terminals of the voltage source. An element terminal is connected to a point between said switches, and a motor phase of the actuator arrangement is connected to the element terminal. A control unit is connected to control the switches in order to charge/discharge the drive elements. A charge control is thereby achieved by the use of the two switches.
In preferred embodiments, inductive motor drive circuits are used, to which the capacitive load of the drive elements is connected. Drive elements may also be connected in parallel. Different resonance circuits are preferably also used.
The present invention changes the mechanical properties of the elements. The present invention enables small sizes and has a low energy consumption. The solutions is also relatively cheap.